1. Field of the Invention
The present invention relates to a display apparatus, a data line driver, and a method for driving a display panel. More specifically, the present invention relates to a technique for collecting and re-using charges accumulated in a data line of a display panel, to thereby reduce the consumed power of a display apparatus.
2. Description of the Related Art
A matrix type display panel on which pixels are arranged in a matrix is a display device of a most typical display apparatus. Typical examples of the matrix display panel include a liquid crystal display (LCD) panel and an organic light emitting diode (OLED) panel. Generally, the matrix type display panel is provided with scan lines each for selecting a pixel row and data lines, to each of which a data signal is supplied based on a pixel gradation. The pixels are arranged at respective positions at which the scan lines and the data lines intersect each other.
Most of power consumption of such a display apparatus is a power used for driving the data lines of the display panel. This is because a capacitance of each data line is inevitably large. The length of each data line needs to be increased according to a size of the display panel. However, an increase in the length of the data line causes an increase in the capacitance of the data line. As a result, the power necessary to drive the data line is undesirably increased. The increase in the power for driving the data line is a serious disadvantage particularly for a liquid crystal display apparatus using an LCD panel. The reason is as follows. Generally, for the liquid crystal display apparatus, an inversion driving method of inverting a polarity of a data signal applied to a pixel is adopted to suppress deterioration of liquid crystal material for the pixels. In other words, pixels are AC-driven. Typically, in whichever a row direction (a scan line direction) or a column direction (a data line direction), a polarity of a data signal supplied to a pixel adjacent to one pixel is inverted. This inversion driving is referred to as “dot inversion driving”. However, to invert the polarity of the data signal, it is necessary to invert the polarity of a voltage of the data line with respect to a reference voltage. This disadvantageously increases the power for driving the data line.
A technique for collecting charges accumulated on the data line in a charge accumulation capacitor is one of effective techniques for reducing the consumed power. Japanese Laid Open Patent Publication (JP-P2001-515225A) discloses a technique for moving charges accumulated in each data line to a charge collecting capacitor, and thereby collecting the charges in the charge collection capacitor in a liquid crystal display apparatus that adopts the dot inversion driving method. FIG. 1 is a circuit diagram showing a configuration of the above conventional liquid crystal display apparatus. In FIG. 1, data lines are driven by even-number column drivers 104 and odd-number column drivers 105. The data lines driven by the even-number column drivers 104 are connected to an even-number reservoir line 216 through even-number coupling transistors 214. The data lines driven by the odd-number column drivers 105 are connected to an odd-number reservoir line 217 through odd-number coupling transistors 215. The liquid crystal display apparatus includes a positive polarity capacitor 220 and a negative polarity capacitor 221 that accumulate charges collected from the data lines. The even-number reservoir line 216 or the odd-number reservoir line 217 can be connected to a desired one of the positive polarity capacitor 220 and the negative polarity capacitor 221 through straight transistors 230 or cross transistors 240. Further, a neutralizing transistor 235 is connected between the even-number reservoir line 216 and the odd-number reservoir line 217. The neutralizing transistor 235 is used to short-circuit the even-number reservoir line 216 and the odd-number reservoir line 217. A reference numeral 110 denotes a capacitance of each data line.
FIGS. 2A to 2D are timing charts showing an operation of the well-known liquid crystal display apparatus disclosed in the above conventional example. FIGS. 2A to 2D show one example of changes in voltage of the data line connected to the even-number column driver 104 and the data line connected to the odd-number column driver 105, respectively. A polarity of each data line is determined in response to a polarity signal POL. In the example of FIGS. 2A to 2D, in a first horizontal period in which the polarity signal POL is at low level, the data line connected to the even-number column driver 104 is driven to a positive polarity voltage relative to the reference voltage. In addition, the data line connected to the odd-number column driver 105 is driven to a negative polarity voltage relative to the reference voltage.
At the end of driving the data lines in the first horizontal period, charges accumulated in the respective data lines are collected in the positive polarity capacitor 220 and the negative polarity capacitor 221. Specifically, the even-number coupling transistor 214 and the odd-number transistor 215 are turned on. The data lines connected to the even-number column drivers 104 and the data lines connected to the odd-number column drivers 105 are thereby connected to the even-number reservoir line 216 and the odd-number reservoir line 217, respectively. Further, the straight transistor 230 is turned on, whereby the even-number reservoir line 216 and the odd-number reservoir line 217 are connected to the positive polarity capacitor 220 and the negative polarity capacitor 221, respectively. As a result, the charges accumulated in the data lines connected to the even-number column drivers 104 are collected in the positive polarity capacitor 220. In addition, the charges accumulated in the data lines connected to the odd-number drivers 105 are collected in the negative polarity capacitor 221. After collection of the charges, the straight transistors 230 are turned off, and the even-number reservoir line 216 and the odd-number reservoir line 217 are disconnected from the positive polarity capacitor 220 and the negative polarity capacitor 221, respectively.
Next, the neutralizing transistor 235 is turned on, and the even-number reservoir line 216 is short-circuited to the odd-number reservoir line 217. The charges of the data lines are thereby neutralized.
In a second horizontal period, polarities of the respective data lines are inverted in response to inversion of the polarity signal POL. Namely, in the second horizontal period, the data lines connected to the even-number column drivers 104 are driven to a negative polarity voltage. In addition, the data lines connected to the odd-number column drivers 105 are driven to a positive polarity voltage. Prior to driving the data lines, the charges accumulated in the positive polarity capacitor 220 and the negative polarity capacitor 221 are re-used for driving the data lines. Specifically, the cross transistors 240 are turned on in response to activation of a latch signal STB. The even-number reservoir line 216 is connected to the negative polarity capacitor 221, and the odd-number reservoir line 217 is connected to the positive polarity capacitor 220. The charges of the positive polarity capacitor 220 and those of the negative polarity capacitor 221 are thereby moved to the data lines connected to the odd-number column drivers 105 and to the data lines connected to the even-number column drivers 104, respectively. That is, the charges accumulated in the positive polarity capacitor 220 and those accumulated in the negative polarity capacitor 221 are re-used to drive the data lines connected to the odd-number column drivers 105 and the data lines connected to the even-number column drivers 104, respectively.
As can be seen, by collecting the charges accumulated on the data lines into the capacitors and re-using the charges, the liquid crystal display apparatus shown in FIG. 1 can effectively reduce the power consumption.
The Japanese Laid Open Patent Publication (JP-P2001-515225A) also discloses the following technique. In order to perform charge accumulation more efficiently, i.e., perform collection and re-using of the charges efficiently, the liquid crystal display apparatus also includes the even-number coupling transistors 214 and the odd-number coupling transistors 215 for connecting specific data lines to corresponding reservoir lines 216 or 217 by using corresponding pixel data, and a decision circuit that decides whether and when to assert a neutralizing signal (in the above reference, FIG. 6 and paragraph [0067]). The Japanese Laid Open Patent Publication (JP-P2001-515225A) further discloses a technique for performing the charge accumulation further efficiently by using not only the pixel data but also additional information indicating voltage levels of the respective capacitors (in the above reference, FIG. 7 and paragraph [0068]).
However, the Japanese Laid Open Patent Publication (JP-P2001-515225A) discloses only abstractly an operation performed by the decision circuit for further efficiently performing collection and re-using of charges. In addition, the Japanese Laid Open Patent Publication (JP-P2001-515225A) fails to disclose how the decision circuit processes the pixel data. Further, the Japanese Laid Open Patent Publication (JP-P2001-515225A) fails to disclose in which case the charge collection and re-using are performed in detail. According to studies of the inventor of the present invention, optimization of an operation for selecting a data line for which the charges are collected and re-used is important for simplifying the circuit.